This invention relates to a redox flow battery system for electrical energy storage and hydrogen production.
There is a clear need for new and sustainable power production technologies, which produce green emissions, negligible waste, are low cost, efficient and suitable for a wide range of geographical conditions. Technology such as this that is already widely commercialised includes photovoltaic panels and wind farms. A major setback in the widespread use of photovoltaics and wind-based power sources is the variable and unpredictable production of electricity, owing to a direct dependence on sunlight or wind. Intermittent energy production prevents a supply and demand routine, with high currents difficult to produce on demand at peak times. For a future “smart grid” it is therefore as important to develop new technology for large-scale energy storage, as it is to efficiently harness renewable energy so as to fully integrate intermittent and green energy production. Large-scale storage systems would already be beneficial for “load levelling”, i.e. the combination of the storage of energy produced but not immediately consumed, as, for instance, nuclear energy produced during the night when energy demand is at its lowest, with its utilization during high demand periods.
Redox flow batteries (RFBs) have been proposed for large-scale energy storage systems. RFBs do not require specific geographical siting, or extensive maintenance, they have a long lifetime, easy implementation and they are resistant to charge and discharge microcycles. The main disadvantage of RFBs is that energy storage capacity is determined by the solubility of the actives electrolytes and that large tanks are thus usually required. They may be applied to load levelling, wind farms and photovoltaic panels [Ponce de León, C., Frías-Ferrer, A., González-García, J., Szánto, D. A., and Walsh, F. C. (2006). Redox flow cells for energy conversion. Journal of Power Sources 160: 716-732]. A RFB is a system in which two half-cells of the battery are each connected to a storage tank containing a solution composed of the redox species and the supporting electrolyte. An ion-exchange membrane separates the half-cells, whilst allowing ion transfer and electrical continuity during charging and discharging of the cell. During charging, the negatively polarised electrode is the cathode, and the electrolyte, in which the redox species is being reduced, is termed the catholyte. Correspondingly, the positive half-cell has the anode and the electrolyte termed the anolyte. Pumps are used to transfer electrolytes from their respective storage tanks to the electrochemical cell, and to the storage tanks again. The key to the system is in choosing appropriate redox species. Generally the redox couples and the electrodes are selected to achieve close-to-reversible kinetics, allowing higher voltage and energy efficiencies of the battery.
Since the study of the first RFB in 1973 a variety of redox species have been used in the catholyte and anolyte [Bartolozzi, M. (1989). Development of redox flow batteries. A historical bibliography. Journal of Power Sources 27: 219-234], and consequently a wide range of RFBs have been patented [e.g. U.S. Pat. No. 4,882,241, U.S. Pat. No. 4,469,760]. The vanadium redox flow battery (VRFB) is a particular case, as it works with vanadium species in both half-cells: the redox couple V(III)/V(II) on the cathodic side, and the redox couple V(V)/V(IV) on the anodic side [U.S. Pat. No. 4,786,567]. It has the advantage that cross-diffusion of cations through the membrane does not affect the cycle current efficiency, and allows for a longer battery lifetime. A VRFB charges when connected to an electricity source, and discharges when connected to an electrical load. The electrolytes flow in one direction through the half-cells, regardless of the process underway, yet the electrochemical reaction taking place differs. The corresponding reactions are:
At the cathode:V3++e−→V2+(charge)  (1a)V2+→V3++e−(discharge)  (1b)
At the anode:V4+→V5++e−(charge)  (2a)V5++e−→V4+(discharge)  (2b)
Since their invention RFBs, and especially all-vanadium RFBs, have been applied to various systems. For instance they have been successfully connected to wind turbines, coupled to solar panels [U.S. Pat. No. 6,005,183], turned into a biofuel powered fuel cell [U.S. Pat. No. 5,660,940], assembled into a stack for a load levelling application [U.S. Pat. No. 7,820,321], and connected to a system that electrochemically regenerated both degraded electrolytes [U.S. Pat. No. 4,956,244].
Hydrogen gas is becoming increasingly important in energy production and consumption management, and is considered as a potential means for energy storage as it can be used as a clean fuel for electricity production. Indeed, in a fuel cell, H2 and O2 gases react to generate electricity and water as the only products. The predominant methods of obtaining hydrogen are steam methane or coal reforming, gasification, and alkaline water electrolysis over nickel electrodes [http://www.hydrogen.energy.gov/pdfs/doe_h2_production.pdf]. The first two processes have the drawback of evolving CO2, and electrolysis cannot be applied intermittently as the nickel electrodes degrade rapidly due to open-circuit corrosion. More sustainable systems with longer lifetimes are therefore needed for H2 production, and some alternatives are already under development [http://www.hydrogen.energy.gov/pdfs/roadmap_manufacturing_hydrogen economy.pdf].
One major aspect in the study of hydrogen evolution is catalysing the reaction, but the most efficient catalyst known to-date is platinum. Platinum is rare and expensive, and is therefore pushing manufacturers and researchers to seek alternative, lower cost, abundant, stable, and equivalently efficient catalysts. One such catalyst is molybdenum sulfide, which has been reported to be an efficient catalyst for H2 evolution. This catalyst has been primarily used in the hydrodesulfurisation reaction in refinery industries, but is now attracting attention for the catalysis of the reaction of hydrogen formation. [Merki, D., Fierro, S., Vrubel, H. and Hu, X. L. (2011) Amorphous molybdenum sulphide films as catalysts for electrochemical hydrogen production in water. Chemical Science 2(7) 1262-1267; Li, Y., Wang, H., Xie L., Liang, Y., Hong, G., and Dai, H. (2011) MoS2 nanoparticles grown on graphene: an advanced catalyst for hydrogen evolution reaction. Journal of American Chemical Society 133 (19) 7296-7299]. This heterogeneous catalyst may be deposited on silica [Rivera-Muñoz, E., Alonso, G., Siadati, M. H., and Chianelli, R. R. (2004). Silica gel-supported, metal-promoted MoS2 catalysts for HDS reactions. Catalysis Letters 94 (3-4):199-204; An, G., Xiong, C., Lu, C., and Chen, Z. (2011). Direct synthesis of porous molybdenum disulfide materials using silica sol as template. Journal of Porous Materials 18: 673-676], should it need to be separated from the products, as for instance, where a fixed or fluidized catalytic bed is used.